﻿76 
  ME. 
  P. 
  E. 
  C0WPEE 
  EEED 
  ON 
  THE 
  [Feb. 
  1 
  897, 
  

  

  nally, 
  the 
  characters 
  of 
  these 
  varieties 
  appear 
  to 
  be 
  identical. 
  The 
  

   small 
  eubcircular, 
  flattened 
  forms 
  which 
  Davidson 
  figures 
  (op. 
  supra 
  

   .eit. 
  fig. 
  25), 
  and 
  which 
  I 
  have 
  also 
  found 
  at 
  Keisley, 
  are 
  probably 
  

   only 
  young 
  individuals 
  of 
  the 
  species. 
  In 
  these 
  the 
  median 
  dorsal 
  

   fold 
  is 
  more 
  sharply 
  defined 
  posteriorly 
  than 
  in 
  the 
  other 
  forms. 
  

   The 
  description 
  of 
  the 
  species 
  is 
  as 
  follows 
  : 
  — 
  Shell 
  longitudinally 
  to 
  

   transversely 
  ovate, 
  subglobose. 
  Hinge-line 
  curved. 
  Cardinal 
  angles 
  

   rounded. 
  Pedicle-valve 
  the 
  larger, 
  with 
  prominent 
  beak 
  incurved 
  

   over 
  the 
  hinge-line 
  ; 
  a 
  weak 
  median 
  sinus 
  is 
  present 
  in 
  this 
  valve 
  — 
  

   narrow 
  near 
  the 
  beak, 
  but 
  increasing 
  in 
  width 
  and 
  depth 
  anteriorly 
  ; 
  

   a 
  slight 
  fold 
  borders 
  it 
  on 
  each 
  side, 
  and 
  in 
  some 
  individuals 
  a 
  faint 
  

   median 
  ridge 
  occurs 
  in 
  it. 
  Brachial 
  valve 
  less 
  convex 
  than 
  the 
  

   ventral 
  ; 
  beak 
  small, 
  and 
  hidden 
  beneath 
  the 
  incurved 
  beak 
  of 
  the 
  

   other 
  valve. 
  A 
  low 
  subquadrate 
  fold 
  with 
  steep 
  sides 
  exists 
  in 
  this 
  

   valve 
  and 
  corresponds 
  to 
  the 
  sinus 
  in 
  the 
  pedicle-valve 
  ; 
  the 
  fold 
  is 
  

   indistinct 
  posteriorly 
  in 
  the 
  adult 
  and 
  old 
  individuals, 
  but 
  strongly 
  

   developed 
  on 
  the 
  anterior 
  edge, 
  where 
  a 
  deep 
  sinus 
  borders 
  it 
  on 
  

   each 
  side. 
  

  

  The 
  surface 
  of 
  the 
  valves 
  is 
  smooth, 
  or 
  decorated 
  with 
  very 
  

   delicate 
  radiating 
  striae 
  (?). 
  

  

  A 
  pair 
  of 
  short, 
  subparallel, 
  dental 
  lamellae 
  is 
  seen 
  in 
  the 
  urn- 
  

   tonal 
  cavity 
  of 
  the 
  pedicle-valve, 
  and 
  a 
  strong 
  muscular 
  impression 
  

   lies 
  between 
  them. 
  

  

  In 
  the 
  brachial 
  valve 
  a 
  short 
  septum 
  exists, 
  extending 
  for 
  about 
  

   one-fifth 
  of 
  the 
  length 
  of 
  the 
  valve. 
  The 
  crural 
  plates 
  (seen 
  in 
  the 
  

   course 
  of 
  developing 
  one 
  specimen) 
  are 
  triangular, 
  and 
  divided 
  by 
  

   a 
  narrow 
  deep 
  median 
  cleft 
  exactly 
  as 
  figured 
  in 
  Hyattella 
  congesta 
  

   (Conr.). 
  1 
  The 
  crura 
  themselves 
  are 
  short, 
  as 
  in 
  that 
  species 
  also. 
  

   The 
  primary 
  lamellae 
  run 
  forward 
  with 
  a 
  steady 
  divergence 
  of 
  from 
  

   25° 
  to 
  30°, 
  with 
  no 
  outward 
  and 
  only 
  a 
  slight 
  upward 
  curvature. 
  

   The 
  points 
  and 
  manner 
  of 
  attachment 
  of 
  these 
  lamellae 
  have 
  not 
  

   been 
  seen. 
  They 
  extend 
  forward 
  for 
  more 
  than 
  two-thirds 
  of 
  the 
  

   length 
  of 
  the 
  valve, 
  and 
  then 
  are 
  coiled 
  into 
  the 
  spiral 
  cones. 
  

   These 
  cones 
  consist 
  of 
  ouly 
  four 
  or 
  five 
  volutions, 
  and 
  form 
  very 
  

   loose 
  coils. 
  Their 
  bases 
  are 
  subparallel 
  to 
  the 
  longitudinal 
  axis 
  

   the 
  shell, 
  and 
  their 
  apices 
  are 
  directed 
  laterally 
  and 
  slightly 
  back- 
  

   ward 
  and 
  downward 
  into 
  the 
  pedicle-valve. 
  

  

  The 
  loop 
  has 
  not 
  been 
  satisfactorily 
  or 
  clearly 
  exposed 
  ; 
  but 
  it 
  

   seems 
  to 
  arise 
  at 
  a 
  point 
  about 
  halfway 
  along 
  the 
  primary 
  lamellae, 
  

   and 
  to 
  make 
  a 
  very 
  acute 
  angle 
  with 
  them, 
  running 
  backward 
  

   with 
  an 
  upward 
  curvature. 
  At 
  the 
  angle 
  where 
  its 
  lateral 
  branches 
  

   unite 
  a 
  strong 
  single 
  median 
  process 
  — 
  apparently 
  tubular 
  — 
  is 
  given 
  

   off, 
  and 
  rises 
  abruptly 
  in 
  an 
  upward 
  direction 
  to 
  the 
  level 
  of 
  the 
  

   primary 
  lamellae. 
  The 
  point 
  of 
  this 
  process 
  is 
  seen 
  as 
  a 
  central 
  dot 
  

   between 
  them 
  in 
  grinding 
  down 
  the 
  brachial 
  valve. 
  

  

  millim. 
  millim. 
  

  

  Length 
  of 
  shell 
  10 
  9 
  

  

  Breadth 
  „ 
  15 
  8-5 
  

  

  1 
  Hall 
  & 
  Clarke, 
  « 
  Pal. 
  N. 
  Y.,' 
  vol. 
  viii. 
  Brach. 
  pt. 
  ii. 
  (1894) 
  p. 
  61, 
  pi. 
  x 
  l, 
  

   figs. 
  23-28. 
  

  

  